The tensor integral of a massless one-loop two-point function is given by (see e.g. Ref. [1]})
\(\int \frac{d^d k}{(2\pi )^d}\frac{k^{\mu _1}\dots k^{\mu _N}}{(-k^2)^{n_1}(-(k-q))^{n_2}} =\frac{i}{(4\pi )^{d/2}}(-q^2)^{d/2-n_1-n_2} \\[5pt]\sum _{r=0}^{[N/2]}\frac{\Gamma (n_1+n_2-r-d/2)\Gamma (d/2+N-n_1-r) \Gamma (d/2-n_2+r)}{2^r \Gamma (n_1) \Gamma (n_2)\Gamma (d+N-n_1-n_2)}(q^2)^{r} \lbrace [g]^r [q]^{N-2r}\rbrace ^{\mu _1 \dots \mu _N} ,\) 
